1. Field of the Invention
The present invention relates to the field of ultrashort pulse duration laser systems suitable for material and biological tissue processing, and more specifically, it relates to the use of a parametric amplifier in ultrashort pulse duration laser systems.
2. Description of Related Art
U.S. Pat. No. 5,720,894, incorporated herein by reference, teaches a method of material processing with high repetition rate, ultrashort-pulse lasers. The patent discloses that any material can be machined with micron scale precision with essentially no thermal or shock damage to the remaining material. A general laser apparatus and method are described which enable this high precision machining performance. A particular embodiment of a laser apparatus utilizing Titanium-doped sapphire as the laser material is described in this reference.
Most laser systems capable of achieving high repetition rate ( greater than 100 Hz), ultrashort-pulse duration ( less than 1 psec) and moderate pulse energy ( greater than 0.1 mJ) realized to date are based on Titanium-doped sapphire as the laser gain material. Recently, a few systems employing Ytterbium-doped materials and Neodymium-doped glass as the laser gain material have been introduced which achieve picosecond and subpicosecond pulse durations and repetition rates in the range 0.1 to 10 kHz. These systems employ a technique known as chirped-pulse amplification in which a low energy, ultrashort laser pulse is stretched in time prior to amplification. The stretched pulse is amplified by a factor of xcx9c106-107 in a high gain amplifier (typically a regenerative amplifier) and then recompressed to near its original pulse duration. A regenerative amplifier is a complex, multipass laser amplifier that requires sophisticated, high-speed electro-optical systems for pulse switching.
Each of these approaches exhibit problems that limit their use for materials processing applications. For Titanium-sapphire systems, the low upper-state lifetime (2 xcexcsec) of the laser material in the high gain amplifier requires that the material be excited (pumped) by a secondary laser. This requirement of a secondary laser system results in a significant increase in the complexity of the system and makes the system sensitive to alignment between the pump laser and the high gain amplifier. Furthermore, the overall electrical to optical conversion efficiency of the system is very low.
For Neodymium-doped glass and Ytterbium-doped materials, the low gain cross section results in the requirement that the laser system be operated at either high fluence ( greater than 4 J/cm2) or in a multipass configuration (such as a regenerative amplifier) for efficient extraction of the energy stored within the laser medium. In chirped-pulse amplification laser systems, this requirement results in a high peak power density ( greater than 5 GW/cm2) on the optics within the laser that often results in damage to the optical components.
In all these systems, achieving moderate average power output ( greater than 1 W) with high beam quality is complicated by thermal distortion and depolarization associated with temperature gradients within the laser material. These gradients result from the residual energy left in the material from the quantum defect between the energy of the photons that excite the laser medium and the energy of the photons in the extracted laser beam.
U.S. Pat. No. 5,400,350, titled xe2x80x9cMethod And Apparatus For Generating High Energy Ultrashort Pulses,xe2x80x9d incorporated herein by reference, is directed to providing compact systems and methods for producing and amplifying chirped pulses to provide high-energy ultrashort pulses. The invention is further directed to providing a compact system that is reliable and cost effective to fabricate. The patent seeks to show that relatively compact tunable lasers can be used to directly generate long chirped optical pulses. For example, compact monolithic semiconductor lasers, which have relatively small dimensions (e.g., dimensions smaller than large frame solid state lasers) and which permit their emission wavelength to be tuned (i.e., changed) relatively fast during the generation of an optical pulse, can be used. These pulsed sources eliminate any need for bulk components in the cavity of, for example, a mode-locked femtosecond laser. The relatively long chirped optical pulses can subsequently be amplified in plural amplification stages to increase their energy content. The patent seeks to show that amplification in plural stages can be achieved by suppressing spontaneous emission between the stages. Afterwards, the amplified pulses can be recompressed. The reference teaches the use of a quasi-phase matched optical parametric amplifier based on periodically poled lithium-niobate for amplifying chirped pulses. This is an alternative approach to the use of high gain laser amplifiers in chirped-pulse laser systems. High gain is achieved by the use of quasi-phase matching. However, the use of periodically poled lithium niobate limits the system to relatively low average power ( less than 1 W) and low pulse energy (typically less than 0.5 mJ).
It is an object of the present invention to provide a method and apparatus for increasing the energy of chirped laser pulses to an output in the range 0.001 to over 10 millijoules at a repetition rate 0.010 to 100 kHz by using an optical parametric amplifier comprised of bulk crystalline material exhibiting a nonlinear susceptibility.
It is another object of the present invention to provide a method and apparatus for increasing the energy of chirped laser pulses to an output in the range 0.001 to over 10 millijoules at a repetition rate 0.010 to 100 kHz by using a double-pass optical parametric amplifier comprised of a single nonlinear crystal; wherein the pump and signal beam size can be independently adjusted on each pass through the crystal.
Still another object of the invention is to achieve the above objects through the use of nonlinear crystals made of nonlinear material from a family of borates including Beta-Barium borate, lithium borate and others exhibiting a second order nonlinear susceptibility.
Another object of the invention is to achieve the above objects through the use of nonlinear crystals made of nonlinear material from a family of phosphates including potassium-dihydrogen phosphate (KDP), potassium titanyl phosphate and others exhibiting a second order nonlinear susceptibility.
These and other objects will be apparent based on the disclosure herein.
The invention is a laser machining system employing an optical parametric amplifier capable of producing pulses at repetition rates in the range 0.1 to 100 kHz with pulse durations in the range 0.01 to 20 psec. By utilizing a bulk nonlinear material and a short-pulse ( less than 3 nsec) pump laser, pulses with energy in the range 0.01 to 10 millijoules can be produced from a compact ultrashort-pulse laser machining system.
Bulk optical parametric amplifiers have not been considered to date for moderate to high average power, ultrashort-pulse applications. The present optical parametric amplifier system does not rely on quasi-phase matching and can achieve both high average power and high gain for broad bandwidth chirped-pulses from a single or double stage system. By relying on parametric conversion rather than conventional laser amplification, there is no residual energy left within the gain medium. As a result, there are negligible thermal gradients and hence, one eliminates the depolarization and beam distortion problems that severely impact the beam quality and electrical to optical conversion efficiency of high average power ultrashort-pulse lasers. In addition to eliminating many of the thermal management problems associated with the high gain amplifier, the use of a parametric amplifier enables the production of the necessary ultrashort duration pulses from a simplified and more compact system. The pulses exiting the parametric amplifier may be compressed directly and used for machining or surgery or may be further amplified in a conventional laser amplifier to provide additional pulse energy before compression.
Any ultrashort-pulse laser machining system could employ the invention to produce a simplified and more compact laser system with improved thermo-optical performance. The invention is a critical step toward realizing compact, and industrially hardened ultrashort-pulse lasers for machining and medical (e.g., surgical) applications.